Ludogorets goalkeeper Milan Borjan has set his sights on winning a move to England in the New Year.
Bolton have completed the capture of former Crewe striker Max Clayton on a three-year contract.
The 25-year-old, who joined the club at the age of 11, has committed himself to another two years.
"I've been here since 1999 and Bayer have improved a great deal in this time," he said.
"The club gives the players the best possible conditions and great opportunities.
"I think we can still achieve a great deal with Leverkusen."
The midfielder has already made 232 Bundesliga appearances for Leverkusen, scoring 18 goals.
"He has taken another leap in terms of quality this season," said the club's director of sport Rudi Voller.
"Of course his consistently good performances meant he had other options, which makes us even more pleased that Gonzalo has opted for us once again.
"We want to build the Leverkusen of the future also around him."
Castro, who has been capped five times by Germany, was part of the Leverkusen side which won the DFB-Pokal in 2009.